Re: Neck turning vs neck reaming
With all due respect to Derek and Gene, I think they may have missed the key point in your question; what's the difference? Yes, reaming can be done to remove the donuts if or when they form, but that's not the primary motivation for this process. Reaming is most often performed when cases are reformed, to thin the necks down to a usable level. I don't think it sees nearly as much use these days as it has in the past for this purpose, due to better availability of "correct" brass today. This wasn't the case some years ago.
While I was in the army, I reformed virtually all of my 243 Win rounds from the tons of 7.62 NATO brass I had available to me. In necking the cases down that far, the necks were invariably far too think to allow for proper neck clearance. As a result, I had to ream the necks down to an appropriate thickness for safety reasons. The reason neck turning is still going strong is because it's an aide to accuracy, whereas reaming is (as I said) a safety/clearance issue, usually used when reforming cases . When you ream necks, you remove an equal amount of brass from all sides of that neck. In other words, if you have non-concentric necks when you begin, when the cases are reamed, you now have thinner necks that are still non-concentric. Neck turning removes neck material unevenly, taking more off the high side and less (or none) off the low side. When you're done with neck turning, you now have (hopefully) slightly thinner necks that are even, concentric and uniform, thus aiding in accuracy.
So why ream instead of just turning everything? Because it's a faster process if you're removing a lot of material. Ideally, you would ream to close to the finished thickness, then cap it off by outside turning to ensure concentricity. As I said, you don't see this as much these days, but it wasn't all that long ago that GI 30-06 brass could be had very, very cheaply, and it got made into everything from 22-250s, .243s, 25-06s, you name it. If it had a .473" head diameter, the price made it worth the extra work. Did my fair share of it then, but I'd really rather open up a box of the "proper" brass today and avoid the hassle altogether!